RED-S: An Athletes Nightmare

March 3, 2022
Featured image for “RED-S: An Athletes Nightmare”

What is RED-S?


RED-S, the acronym for Relative Energy Deficiency in Sport, applies to a syndrome that can lead to a range of negative health outcomes, says Sport Scientist Canada. These effects on health and performance are largely caused by a prolonged period of under fueling – athletes are consuming less energy than they are expending. Chronic low energy availability in athletes means that the body cannot support other important bodily functions including growth, recovery, menstruation, adaptation to training and bone development.


“We’ve seen (RED-S) in sprint power track athletes, and weightlifting-type sports…”


Athletes can develop RED-S as a result of eating disorders, through failed attempts to manage optimal body composition, and also due to a lack of  appropriate caloric intake with increasing energy expenditure (i.e., training).




Athletes of all genders and sports are at risk of developing RED-S.Kate Ayers Red-S Article

“Classically, you would assume it would be any sort of athlete (who) is in a weight class sport or a body image judged sport, or endurance athletes who are trying hard to have a good strength-to-weight ratio,” says Alexandra Coates, an exercise physiology PhD candidate at the University of Guelph. “Any kind of sport that has a weight factor in it, will be at higher risk (of RED-S) than other sports.”

For example, elite runners demonstrating significant risk factors for RED-S are 4.5 times more likely to experience a stress fracture than those who do not demonstrate RED-S risk factors.

However, athletes who are not aiming to cut weight can dip into a RED-S state. “It can often occur just from training load. People are training so hard that they aren’t taking in enough calories without even realizing it,” says Coates. “We’ve seen (RED-S) in sprint power track athletes, and weightlifting-type sports because they are training so much that they haven’t quite matched their training load with their eating.”




Kate Ayers - Red-S Article SymptomsRED-S can present itself in different ways depending on gender and severity of energy deficiency.

For menstruating individuals who are not on a hormonal birth control, cycle disruptions or loss of periods is a tell-tale sign of RED-S.

“But because 50 per cent of the population is non-menstruating and another 25 per cent is on hormonal birth control, there are a lot of other symptoms you can look for,” Coates says.

“Initially in an energy deficit, you’re going to have some weight loss. You’re going to have a decrease in resting heart rate and decrease in blood pressure, which is driven by some hormone changes. Sometimes all these things are misinterpreted as good signs, … as ‘I’m getting fitter.’” But these physiological changes could signal RED-S.

Athletes with RED-S may also experience training inconsistencies because of a lack of fuel to support training, which means that muscle glycogen is suboptimal.

An unexpected potential sign is weight gain.


RED-S can cause your body to lower metabolic rate, to stop losing weight.”


“One of the key features of RED-S is because you’re in an energy deficit, your body goes into a protection starvation mode and will start lowering resting metabolic rates so that you stop losing weight,” Coates says. “People will be in an energy deficit, but they will not be losing weight and some cases will see weight gain.”

For athletes who are concerned about RED-S, they should ideally seek out a sports medicine physician for follow-up.

“Blood markers will show up. … With low energy availability, you’re going to see changes like decreased thyroid hormone, decreased ferritin or iron, increased cortisol, decreased IGF-1 which is a growth hormone, and decreased sex hormones like testosterone and estrogen,” Coates says. “A normal blood panel from your doctor some things will show up as abnormal and that could be another sign that you are in an energy deficit.”

Stress fractures and other bone injuries are also common in people who are in a prolonged energy deficit.




Initial weight loss can sometimes cause a temporary boost in performance for endurance-based athletes, but continued weight loss and improvement isKate Ayers Red-S Article unstainable. Individuals who are under fuelled do not have enough reserves to recover from training sessions.

“The big thing we are seeing on the performance side is that there is this failure to adapt to training in a way you’d normally expect,” Coates says. “What we found with people in RED-S is that because their bodies are just trying to get back to normal and healthy states, you’re not going to improve from the training. You may not see an impairment or underperformance, but you aren’t improving from training because you don’t have the resources.”

Those with RED-S are at a greater risk of injury and illness, which both lead to decreased performance. RED-S can also have longer-term negative implications for reproductive, cardiovascular and bone health.




Kate Ayers Red-S Article, HULK and Informed ChoiceAwareness is a solid first step in preventing RED-S in athletes.

“If we can know about it, that might help prevent some of the more long-term effects, like hormonal issues and decreases in bone mineral density, which is so hard to recover from,” Coates says.

Athletes can work with sports dieticians and other specialists to help ensure that their energy intakes are adequate for training and recovery.

One way for athletes to ensure that they are recovering from training sessions is to consume protein and carbohydrates within 30 minutes after their workouts. PERFECT SPORTS HULK Clean Mass Gainer offers a convenient, clean and delicious way to consume the protein and micronutrients needed for muscle recovery.

Kate Ayers Team Canada Runner, PERFECT Sports sponsored athlete and writer

Kate Ayers – Author

Kate grew up on a beef and cash crop farm in Simcoe County, Ont. She completed a degree in agricultural science at the University of Guelph while competing in athletics internationally. Kate combined her passions for agriculture, sport and storytelling by becoming a freelance writer. Now in Victoria, she’s an Athletics Canada 1500m runner training for the Olympics.